Description

While testing sugar using openfire as Jabber server, I noticed Gabble was confused by echoed pubsub notifications making shared activities not appearing on the mesh view.
Ejabberd doesn't send these notifications so we didn't notice this before (seems it's an ejabberd bug according to XEP-0163).

Attached a patch (already reviewed and merged upstream) fixing that. I think we should ship it to make sharing work with openfire and avoid regressions if we upgrade our ejabberd server to a version which does send these notifications.